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Types of Electrical
Overstress Protection
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Introduction
ON Semiconductor makes a variety of silicon based
protection products, including standard and Zener diode
based Transient Voltage Suppressors (TVS), Thyristor
Surge Protection Devices (TSPD) and Electronic Fuses
(eFuse). TVS devices are also included as built in protectors
in our line of single ended filters and common mode filters.
Protection devices are made in a variety of other
technologies, however. This application note will describe
some of the most popular components used to protect
systems from electrical over stress and will begin with
a general discussion of the general classes of protection
products and how they work. That will be followed by
sections on voltage limiting and current limiting protection
and descriptions of the various technologies available along
with a summary of their strengths and weaknesses.
APPLICATION NOTE
Figure 1. Illustration of How Protection Devices Limit
Stress to Sensitive Components
Protecting Electrical Systems
Sensitive electrical components in electronic systems are
protected from electrical overstress in a number of ways.
The first line of defense is the system’s case which provides
physical protection. The second line of defense is proper
grounding which shunts excess voltages and currents away
from sensitive nodes. Systems, however, need inputs and
outputs such as keyboards, video outputs and data interfaces
such as USB ports. These input and output ports often
provide an easy path for electrical overstress during normal
use. Protection products are used on input and output lines
to limit electrical stress from the most commonly occurring
stresses, which range from electrostatic discharge through
lightning.
Protection components fall into two categories, voltage
suppression and current limiting. These are illustrated in
Figure 1 in which a voltage limiting device is represented by
a diode and a current limiting device is represented by a fuse.
Voltage limiters are placed between a sensitive node and
a low impedance voltage, usually ground. During normal
operation of the system the voltage limiting component must
have high impedance. Outside of normal operating voltages
the voltage limiting device must switch to a low impedance
to shunt stress current to ground, limiting the voltage on the
sensitive node. Current limiting devices must be placed in
line with the signal. Current limiters must have low
resistance during normal system operation but switch to
a high impedance state during a stress condition.
© Semiconductor Components Industries, LLC, 2014
June, 2014 − Rev. 1
Voltage Suppressors
All integrated circuit pins have a range of voltage and
current over which they are designed to operate. Above the
normal operating voltage is a region of safe overvoltage,
which will not cause damage to the circuit. At higher
voltages the integrated circuit will be damaged. These
regions are illustrated in Figure 2. The boundary between
safe overvoltage and device damage is not sharp. Higher
voltages can often be tolerated if the voltage excursion is of
short time duration. Voltage suppressors need to work in the
region of safe overvoltage of the circuit they are protecting
by preventing voltage from entering the device damage
region, but not degrade system performance within the
range of normal operation.
Voltage suppressors are classified by protection behavior
and by their directionality. The protection behavior
differentiates between voltage clamping and snapback
while directionality differentiates between unidirectional
and bidirectional.
Voltage suppressor devices fall into two categories of
behavior, voltage clamping and snapback protection as
illustrated in Figure 2. A voltage clamping device has high
resistance up to a turn on voltage, above which the resistance
drops dramatically. For a voltage clamping device it is
important that the turn on voltage is above the normal
operating voltage of the system but low enough to ensure the
clamping voltage is well below the damage voltage for the
node being protected. The on state resistance must also be
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extremely low to ensure low clamping voltages across
a range of conditions that the electrical threat can present to
the circuit.
A snapback device behaves differently. A snapback
device also has high resistance at low voltage and turns on
at a voltage above the normal operating voltage of the circuit
being protected. In a snapback device however, the turn on
voltage initiates a new conduction mechanism which creates
a low resistance and the voltage drops well below the turn on
voltage. In some instances the voltage may actually drop
into the normal operation range. As illustrated in Figure 2
snapback devices may allow brief transients into the device
damage region before returning to the safe voltage range.
For many circuits such brief transients are acceptable since
the onset of damage is often very time dependent.
Some snapback type devices are referred to as crowbar
devices. A crowbar device has a snapback type of I−V curve
but has a very low on state resistance and can carry large
currents making them suitable for surges caused by
lightning and power line disturbances. Thyristor Surge
Protection Devices (TSPD) and Gas Discharge Tubes
(GDT) are often called crowbar devices. Polymer surge
protection devices do not have a very low on state resistance
and cannot be classified as crowbar devices.
Voltage limiting devices are also classified as
bidirectional or unidirectional. Sample I−V curves for
bidirectional and unidirectional voltage clamping
protection components are shown on the left side of
Figure 3. A bidirectional protection device has symmetrical
protection properties centered at zero volts. As illustrated in
the upper right part of Figure 3 a bidirectional protection
structure is ideal for protecting nodes whose voltage
normally extends symmetrically above and below zero
volts, such as audio line applications. A unidirectional
device has asymmetric behavior around zero volts. A classic
unidirectional protection device is a Zener diode. In the
forward direction the Zener begins to conduct strongly at
about 0.7 V. In the reverse bias direction the Zener begins to
conduct at its reverse bias breakdown voltage. As shown in
the lower right part of Figure 3, a unidirectional protection
device is ideal for protecting voltage nodes whose voltage is
either always positive or always negative, such as data line
applications. It is a common mistake to assume that
bidirectional protection components protect for both
positive and negative stress while unidirectional protection
elements only protect for one stress polarity. This is not true,
both unidirectional and bidirectional protection products
can protect stress with either polarity.
Figure 2. I−V Curves for Voltage Clamping and
Snapback Voltage Limiting Devices and How They
Relate to the Properties of the Circuits They are
Protecting
Figure 3. Bidirectional and Unidirectional Voltage Clamping Protection Devices
with the Range of Safe and Unsafe Conditions for the Circuit Being Protected
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Diode Transient Voltage Suppressors
Diodes are two terminal circuit elements which conduct
current easily in one polarity and have a high resistance, up
to some breakdown voltage, in the opposite polarity. Today
most diodes are solid state devices made from Silicon.
Figure 4. Diode Protection
capacitance. This made them unsuitable for high speed
applications. Recent technology advances have eliminated
that disadvantage.
New products such as the ESD9L5.0 combine the
advantages of silicon based protection with the low
capacitance required for high speed applications.
The ESD9L5.0 behaves as if it were a simple Zener diode.
In fact the ESD9L5.0 includes a low breakdown Zener diode
and a pair of high breakdown, and therefore low
capacitance, standard diodes as shown in Figure 5. The DC
I−V characteristics of the ESD9L5.0 are the same as for
a low voltage Zener diode. For a negative cathode to anode
voltage the single diode in the right branch provides forward
bias diode conductivity. For positive cathode to anode
voltages the right branch of the ESD9L carries little current,
but the left branch will have a breakdown voltage equal to
the Zener diode’s breakdown plus a forward bias diode drop.
The ESD9L5.0 obtains its low capacitance from the low
capacitance of the two standard diodes. The right branch of
the circuit has the low capacitance characteristic of the high
breakdown diode, while in the left branch the series diodes
will have a capacitance no higher than the capacitance of the
low capacitance high breakdown diode.
Silicon diodes are formed at an interface between n doped
and p doped regions of silicon as illustrated in Figure 4. At
the junction the electric fields between the n and p doped
regions create a depleted region without either n or p carriers
when there is no externally applied voltage. If a negative
voltage is applied from the cathode to the anode both n and
p carriers are pushed across the junction (in opposite
directions) and current begins to flow at about 0.7 V. If
a positive voltage is applied from the cathode to the anode
the n and p carriers are pulled further apart and very little
current flows. At a high enough voltage either avalanche
breakdown or Zener tunneling will result in high currents.
A basic diode’s I−V curve is shown in Figure 4. Diodes are
inherently unidirectional devices and protect by voltage
clamping. The properties of the diode depend on the doping
levels of the n and p regions both near the junction and far
from the junction. Despite the simple nature of the basic
diode structure variations in doping profiles can create
a very wide range of diode properties. Diodes designed for
protection have benefited from all of the advances used in
state of the art silicon technology development.
One of the most important diode properties is the
breakdown voltage. Reverse bias diode breakdown voltages
can vary widely from just a few volts up to hundreds of volts.
Most silicon based diodes with a well defined breakdown
voltage are referred to as Zener diodes. The circuit symbols
for standard and Zener diodes are shown in Figure 4.
(Reverse bias conduction by Zener tunneling is only
significant for diodes with breakdowns of 6 V and less.
Diodes with breakdown voltages above 6 V usually begin to
conduct in the reverse bias direction by avalanche
breakdown. The use of the term Zener diode for these higher
breakdown voltage devices is standard in the industry,
however.)
In the past silicon based TVS devices had a disadvantage
protecting low voltage, high speed signal lines. Lowering
the turn on voltage of a protection diode requires higher
doping levels in the silicon which results in a high
Figure 5. Effective and Actual Schematic of
ON Semiconductor’s ESD9L5.0, Ultra Low
Capacitance Diode Based TVS Device
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Thyristors
Zener diode based Transient Voltage Suppressors are
a very effective products for protection. They have high
conductivity in their on state, their turn on voltage can be
controlled accurately, and they do not degrade with repeated
stresses below their maximum current limits. They can also
be manufactured in a variety of configurations on a single
silicon substrate using integrated circuit processing
techniques, as illustrated in Figure 6. Products such as
ON Semiconductor’s ESD9L3.3 provide protection to
unidirectional signals while bipolar signal lines can be
protected by products such as ON Semiconductor’s
ESD7951. Single package solutions to protect multiple
signal lines are also available with products such as
ON Semiconductor’s ESD7004 which is ideal for USB 3.0
and HDMI applications.
Thyristor surge protection devices are based on a pair of
interconnected bipolar transistors created by a 4 layer stack
of n and p doped silicon regions as shown Figure 7. The n
doped region N1, p doped region P1, and n doped region N2
form the emitter, base and collector of an npn transistor
while p doped region P2, n doped region N2, and p doped
region P1 form the emitter, base and collector of a PNP
transistor. With this arrangement the collector of each
transistor provides the base of the other transistor as in the
center of Figure 7. In this way any emitter to collector
current of one transistor provides the base current for the
other transistor, thereby powering it into the conduction
regime. For a positive Anode to Cathode voltage, both
emitter-base junctions, J1 and J3, are forward biased. Only
the reverse biased junction J2 prevents current flow. If the
Anode to Cathode voltage is increased to the breakdown
voltage of the J2 junction, currents begin to flow directly
into the bases of the two bipolar transistors. This turns both
transistors on. With both transistors on, the thyristor’s
resistance drops, and the voltage across the thyristor also
drops. The resulting I−V curve for forcing a positive current
from the Anode to the Cathode of a thyristor is shown in
Figure 7. A protection element with this form of I−V curve
can provide excellent protection; when triggered the voltage
drops well below the trigger condition and considerable
current can be carried with very little power dissipation in
the protection element. The protection properties have
a snapback behavior for positive voltage. The low on state
resistance of a thyristor makes it appropriate to call them
crowbar devices.
Figure 6. Variety of Diode Based Protection
Products Combining Multiple Diodes
Figure 7. Basic Thyristor Structure and Behavior
junction is providing the emitter for both transistors no
regenerative behavior is possible. For negative Anode to
Cathode bias the Thyristor breakdown looks similar to
a reverse biased diode, as shown in the I−V curve in
Figure 8.
Under a negative Anode to Cathode voltage the situation
is very different. Only the junction J2 is forward biased. This
forward bias junction could be considered the emitter base
junction of a pair of bipolar transistors as shown in an
unconventional depiction in Figure 8. Since the same
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Figure 8. Thyristor Under Negative Voltage
Figure 9b. The integrated device is usually called
a Thyristor Surge Protection Device (TSPD), although
some companies refer to them as SIDACTORs. The I−V
characteristic of a TSPD is shown in Figure 9c.
To provide symmetrical snapback behavior it is necessary
to use two anti parallel thyristors. This can be achieved with
a pair of discrete thyristors, as shown in Figure 9a, or it can
be done with an integrated structure on a single piece of
silicon including five doping levels, as illustrated in
Figure 9. Creation of a Bidirectional Thyristor Protection Device
There are some design constraints that must be considered
when using TSPDs. The holding voltage and current of
a TSPD are defined in Figure 10. The holding voltage is
often below the operating voltage of the circuit being
protected. If the power source for the protected node can
supply the holding current it is possible for a system to go
into a latch-up state after a stress. This condition can be
avoided by choosing a TSPD with holding currents greater
than the system’s power supply can source or by adding
a small amount of resistance in the circuit to lower the
current that can be supplied to a TSPD after a stress event.
Figure 10. Holding Voltage and Current of a TSPD
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resistance drops significantly as shown in Figure 11a.
Varistors are usually made of a ceramic of zinc oxide grains
in a matrix of other oxides as illustrated in Figure 11b.
The grains form diodes with the surrounding matrix,
creating a complex array of parallel and anti-parallel diodes.
At low voltage each miniature diode has a very low voltage
across it and very little current flows. At higher voltages
individual diodes begin to conduct and the resistance of the
varistor drops significantly. Factors such as grain size,
the nature of the matrix material between the grains,
the thickness of the ceramic and the attachment of leads to
the ceramic determine the properties of the varistor. To
obtain low voltage turn on and improve conductivity most
MOV varistors are constructed as a multi layer structure as
shown in Figure 11c.
TSPDs are excellent protection elements under electrical
surge conditions since they can carry large amounts of
current without being damaged and are very popular for
telecommunication protection. They have low leakage and
they come with a wide range of turn on voltages to match
a variety of applications. Recent technology advances have
produced low capacitance TSPDs that can be used to protect
telecommunications circuits carrying high speed xDSL
data.
Metal Oxide Varistors (MOV)
The metal oxide varistor is the first of the
non-silicon-based voltage limiting devices discussed in this
document. The term varistor is a combination of variable
and resistor. At low currents and voltages varistors have
a high resistance, but at higher voltages and currents the
Figure 11. Metal Oxide Varistor and a Symbol Often Used for a Varistor
conducting particles, thereby forming a lower resistance
path and the voltage across the device drops. The breakdown
voltage will depend on the nature of the polymer, the sizes
of the conducting particles and the separation between them.
Polymer surge suppression devices are snapback devices
and are always bidirectional as illustrated by their I−V curve
in Figure 12.
Varistors are always bidirectional devices but are
manufactured with a very wide range of current and voltage
capacities for applications ranging from lightning protection
for high voltage transmission lines to small surface mount
devices intended for ESD protection. Varistors, however,
have a relatively high capacitance relative to their
conductivity. These limitations of varistors result in much
higher on state resistance compared to silicon devices and
thus much higher clamping voltages. Varistors therefore
have limited application in the protection of high speed
signal lines against ESD threats. Varistor performance can
degrade with stresses well below their rated values. In
addition, since the performance of the varistor is dependent
on the material composition within the product, which can
vary from part to part and from stress to stress, the clamping
voltage performance is inconsistent and unreliable during
the electrical stress. A commonly used circuit symbol for
a varistor is shown on the right side of Figure 11.
Figure 12. Polymer Surge Suppressor
Polymer Surge Suppressors
Polymer protection is formed by a volume of polymer
surrounding an array of conducting particles as illustrated in
Figure 12. At low voltages the polymer has a very high
resistance. At higher voltages arcs form between the
Polymer devices have very low capacitance, which can be
attractive for maintaining signal integrity of high speed
applications. They have several down sides, however. They
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curve in Figure 13. GDTs are snapback devices and are
always bidirectional. The on state resistance of GDTs is very
low and they are a classic crowbar device.
Gas Discharge Tubes with three or more electrodes can be
constructed with a single volume of gas by having holes in
the inner electrodes. The result is that an arc formed by
a trigger voltage between any two adjacent electrodes will
result in a low resistance path between all of the electrodes
as the ionized gas fills the entire gas chamber. This can be an
important feature for multi line signal ports which might be
sensitive to large voltages imbalances between the lines. Gas
discharge tubes have a bidirectional snapback I−V similar to
a bidirectional thyristor. The I−V curve is somewhat more
complex due to glow regions which form between the initial
breakdown and a full arc regime.
have high trigger voltages that can exceed 1000 V leaving
sensitive components unprotected for low voltage stress
events. Their on state resistance is poor, which can allow
100s of volts across sensitive circuits during an ESD event
even after the protection device triggers. Polymer surge
suppressors can also degradation under multiple stresses.
Gas Discharge Tubes (GDT)
Gas discharge tubes are usually formed with a ceramic
body filled with a gas mixture containing neon and argon
and two or more electrodes as shown in on the left of
Figure 13. When the voltage across the electrodes exceeds
a specified value a glow discharge is formed and this is
followed by an arc if sufficient current is available, thereby
providing a low current path. This is illustrated in the I−V
Figure 13. Gas Discharge Tube Construction, Electrical Symbols and I−V Curves
protection devices in coordination with other faster turn on
and lower voltage secondary protection elements.
Gas discharge tubes can carry very large amounts of
current and have very low capacitance resulting in very little
signal loading. On the downside, the lowest turn on voltage
for a GDT is about 75 V and the turn on time is relatively
long. GDTs are also relatively large and more expensive than
other surge protection devices. They excel as primary
Summary of Voltage Suppressors
Table 1 provides a summary of voltage limiting protection
devices and their properties while Table 2 outlines each
devices advantages and disadvantages.
Table 1. SUMMARY OF VOLTAGE SUPPRESSION DEVICES
Type
Protection Mechanism
Application
Directionality
Strategy
Transient Voltage
Suppressors (TVS)
Mix of Forward Bias, and Avalanche and Zener
Breakdown Silicon Diodes
Surge and ESD
Unidirectional or
Bidirectional
V Clamp
Thyristor (TSPD)
Turn On of Coupled Bipolar Transistors (SCR)
Lightning & Surge
Usually Bidirectional
Snapback
(Crowbar)
Metal Oxide Varistor
(MOV)
Metal Oxide Non-linear Resistance
Lightning,
Surge & ESD
Bidirectional
V Clamp
Polymer
Arcs in Polymer w. Conductive Particles
ESD
Bidirectional
Snapback
Gas Discharge Tube
(GDT)
Arc in Gas
Lightning & Surge
Bidirectional
Snapback
(Crowbar)
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Table 2. ADVANTAGES AND DISADVANTAGES OF VARIOUS VOLTAGE LIMITING DEVICES
Type
Transient Voltage
Suppressors (TVS)
Thyristor (TSPD)
Metal Oxide Varistor
(MOV)
Polymer
Gas Discharge Tube
(GDT)
Advantage
Disadvantage
Well Controlled Turn On Voltage
Low Resistance in On State
Unidirectional and Bidirectional Versions
Low Capacitance Versions Available
Low Leakage
Fast Response
Limited Power Ratings
High Current Carrying Capability
Low Capacitance Versions Available
Care Needed in Selection & Design to Avoid Latch-up; Otherwise
Power Reset Needed after Surge Event.
Slower Turn On than TVS – Not Fast enough for ESD Protection
Low Cost
Fast Response
High Capacitance
High Resistance in On State; Poor Power Dissipation Capability
Very Low Capacitance
Low Cost
High Turn On Voltage
High Resistance in On State
Very High Current Carrying Capability
Low Capacitance
High Off State Impedance
Large Size
Slow Response
Expensive
Current Limiting Devices
There are four major types of current limiting devices,
fuses, circuit breakers, PTC or positive temperature
coefficient devices and electronic fuses (eFuse). The goal of
each of the devices is to stop current flow if it gets too high.
Each device has its strengths and weaknesses.
the FET and adjusts the FET gate voltage to maintain the
output voltage at a specified maximum value.
Finally the thermal shutdown circuit monitors the
temperature of the FET. If the FET temperature exceeds
a preset value the FET is turned off, removing power from
the load. ON Semiconductor eFuses can be set to
automatically restore power to the load after cool down, or
require a power cycle or toggling of the enable pin to restore
current flow.
Electronic Fuse (eFuse)
An electronic fuse is an active electronic component
which can limit current applied to a load as well as remove
power from the load entirely. Additional features present in
ON Semiconductor eFuses include voltage-clamping of
transients. A block diagram of a basic electronic fuse is
shown in Figure 14. The central element is a large power
FET connecting the power source to the load. The use of
a charge pump for the gate allows the FET to have a very low
series resistance between the power source and the load
during normal operation. Control circuits adjust the charge
pump voltage supplied to the FET during normal,
over-voltage, over-current and turn-on conditions.
Three circuits control the charge pump voltage; current
limit, voltage limit (clamp) and thermal shutdown circuits.
The current limit circuit monitors the voltage across the
current sense resistor RS in Figure 14. The current sense
resistor is connected to a small section of the large FET.
Since the sense resistor is monitoring only a fraction of the
total current it can be a low power surface mount resistor
placed on the board. The value of the sense resistor sets the
“overload current” level above which current will be
clamped; the clamped level is known as the “short-circuit
current”. For a given sense resistor value, the short-circuit
current limit is generally lower than the corresponding
overload-current level, as a result of which the clamping
event leads to a downward transient in the current.
During normal operation the output voltage tracks the
input voltage (VCC), less the drop across the low-RDSON
FET. If the VCC voltage gets too high and exceeds a preset
value, the voltage limit circuit senses the source voltage of
Figure 14. Basic Block Diagram of Primary Electronic
Fuse Functions
There are a number of advantages to electronic fuses over
other current limiting options. Current can be limited to
a preset value, without removing all power to the load.
ON Semiconductor electronic fuses can also limit
overvoltage in the absence of high current conditions and do
so without shutting the system down. Both the voltage and
current limit functions work at the speed of an electronic
circuit, rather than the slower thermal times characteristic of
fusible conductor fuses and positive temperature coefficient
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Circuit Breakers
(PTC) devices or the mechanical cycle times of a circuit
breaker. Finally the thermal function will remove power
from the load in the situation of a long term (sub-millisecond
range) over-current condition. ON Semiconductor eFuses
have additional features such as under voltage lock out,
dV/dt control on power up, logic controlled enable functions
and outputs to signal fault conditions. The enable function
is sometimes used to combine and coordinate multiple
eFuses in parallel and thereby control multiple voltage buses
and achieve higher DC-currents (and correspondingly
higher trip currents). These advantages make them ideal for
high reliability systems that require protection from
over-voltage and over-current conditions with a minimum
of system down time and with high precision.
Circuit breakers are the next class of current limiters.
Circuit breakers employ a mechanical switch that
automatically opens when current is too high.
The mechanical switch can be operated by an
electromagnetic device, as in the conceptual example in
Figure 16, with a bimetallic strip, a combination of the two
or more complex electronic high current sensing circuits.
Typically circuit breakers, once they have been activated,
require a manual reset to return power to the circuit. Circuit
breakers typically can also be used as a power switch in the
absence of an overload current. Parameters that are
important for circuit breakers are similar to fuses, even
though their operation is very different. First of all is
a maximum current for operation without activation,
the maximum voltage that the breaker is intended to block,
a maximum interrupt current and parameters for speed of
activation.
Non-resettable Fuses
The primary element of a traditional non-resettable fuse
is a fusible conductor that melts when current gets too high,
as shown in Figure 15 for a cylindrical fuse. The nature of
non-resettable fuses dictates that they will operate only once
and must be replaced after operation. There are several
parameters which define the capability of fuses. First of all
is the amount of current that the fuse is designed to carry
without opening. Next is the fuse’s rated voltage. After the
melting of the fusible element the fuse must be able to
sustain the full voltage of the electrical supply without
arcing or carrying excess leakage. Less well understood by
people not familiar with non-resettable fuses is the
maximum interrupt current. When a fusible element melts
an arc is usually formed between the two terminals of the
fuse. If the current is high enough this arc may continue to
conduct current, even without the presence of a conductor
between the two fuse terminals. This is especially an issue
for DC circuits where there is no zero voltage crossing to
help extinguish the arc. Another important parameter for
fuses is the time to open. The time to open for a fuse will vary
depending on how high the fault current is. Very high fault
current will cause very quick action but lower fault currents
can take considerable time for the fuse element to melt.
Figure 16. Conceptual Drawing of a Circuit Breaker
Positive Temperature Coefficient (PTC) Device
The final common current limiting device is the PTC or
positive temperature coefficient device. A PTC has low
resistance at low temperature but its resistance rises very
rapidly above a particular current due to self heating of the
resistor. Lowering the temperature returns the PTC to a low
resistance state. Most PTC devices are made from a polymer
containing a high density of conducting particles, often
carbon black as illustrated in Figure 17. The density of
particles is high enough that the conducting particles either
touch or are so close together that conduction occurs through
tunneling. During an over−current event, the current
through the PTC leads to resistive heating and the polymer
goes through a phase change, increasing in volume.
The increase in volume pulls the conducting particles apart,
resulting in a large increase in resistance. Unlike fuses and
circuit breakers PTCs need to continue to carry some current
Figure 15. Construction of a Cylindrical Fuse
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Summary of Current Limiting Devices
to maintain the high resistance state. When the current is
removed the PTC cools and returns to a low resistance state.
PTCs provide a self resetting current limiting device. PTCs
are characterized by the current they are intended to carry in
normal operation, the maximum voltage that they can block,
and the maximum current they can interrupt. PTCs are
affected more by ambient temperature changes than are
non-resettable fuses or circuit breakers.
Table 3 provides a summary of the four types of current
limiting devices and their advantages and disadvantages.
Figure 17. PTC Construction
Table 3. SUMMARY OF CURRENT LIMITING DEVICES
Type
Advantages
Disadvantages
eFuse
Precision
Automatically or Manually Re-settable
Fast Acting
Limits Current without Removing Power for Short
Duration Over-current
Limits Over-voltage
Requires External Resistor
Moderate Cost
Fuse
Wide Range of Parameters
Slow Response
Single Use Device
Circuit Breaker
Can be Reset
Can Double as Switch
Large Size
Expensive
Slow
PTC
Self Resetting
Inexpensive
Slow Acting
Sensitive to Ambient Temperature
Slow to Regain Low Resistive State
Summary
This application note has summarized the basic properties
of some of the most widely used circuit protection elements
in use today. ON Semiconductor’s silicon based solutions
play an important role in both voltage suppression and
current limiting protection. Diode based TVS devices are
the solution of choice for a range of low to medium energy
stresses such as Electrostatic Discharge (ESD) where
ON Semiconductor’s TVS devices have unequalled
capability to combine low capacitance, excellent clamping
and high reliability, all of which are ideal and necessary for
today’s most demanding high speed interfaces. Thyristor
Surge
Protection
Devices
(TSPD)
excel
in
telecommunication protection and ON Semiconductor’s
new low capacitance products are ideal for the higher speed
versions of Ethernet. For current limiting protection
ON Semiconductor’s eFuse line offers current limiting
functions with a range of other features such as current
limiting without shutdown for short duration over-current
excursions, shutdown for longer duration over-current
situations, voltage limiting, low voltage shutdown and auto
reset, and non auto reset options.
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